Method of finishing flat surfaces

ABSTRACT

A method of finishing a flat surface of a work piece which comprises the steps of (1) machining a surface by urging the work piece under pressure toward a revolving plate which is covered with a free-flowing abrasive slurry, and (2) subsequently directing a stream of beads against the machined surface.

United States Patent 1191 1111 39 9 Boettcher 01:11. 9, 1973 [5 METHOD OF FINISHING FLAT SURFACES 3,531,964 10/1970 Manning 72/53 2,239,044 4/1941 Leighton 72/53 UX [75 1 lnvemor- Stephen member, Deel'field 3,643,379 2/1972 Testolin 51/317 x 11L 3,624,967 12/1971 Kamper 51/8 73] As s l g fieef gbaffiiiorfioraffflfi; I56; 2,448,316 8/1948 Lesavoy 51/319 X Plaines, Ill. Primary Examiner-Donald G. Kelly [22] Flled" June 1972 Att0rneyJohn A. Dienner et al. [21] Appl. No.: 260,525

[57] ABSTRACT 52] 11.s.c1 511/317, 51/5, 51/324, A method of finishing a flat surface of a Work piece 51 /326, 72/53 which comprises the steps of (1) machining a surface [51] Int. Cl 1324b 1/00 y rg g the work piece under pr re oward a re- [58] Field 61 Search 51/317, 318, 319, volving plate which is covered with a f e-fl wing 51/321, 323, 324, 326, 5, 8; 72/53 abrasive slurry, and (2) subsequently directing a stream of beads against the machined surface. [56] References Cited 11 Claims, No Drawings UNITED STATES PATENTS 937,180 10/1909 Ridd 72/53 X METHOD OF FINTSHING FLAT SURFACES BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to a method of finishing flat surfaces, for example on the side plates of a Wankel engine, which are exposed to high speed sliding of a mated part on the order of up to 5,000 feet per minute.

2. Description of the Prior Art Heretofore, the flat surfaces of work pieces have been finished in one of two ways: they have been ground, for example, by a reciprocating grinder; alternatively, they have been machined by urging the work pieces under pressure toward a revolving plate which is covered with a free-flowing abrasive slurry.

The resultant finished surface ofa ground work piece may be compared to a mill file or straight knurl, as it is formed by a multiplicity of relatively straight parallel ridges. In some instances there may be two disciplines of ridges at an angle to each other producing a series of peaks. These may be readily likened to a diamond knurl or a cross-cut file. The finished surface of a work piece machined with a free-flowing abrasive slurry similarly exhibits the characteristics of a diamond knurl.

With such surfaces, a theoretically perfectly flat mating part would bear against only the crests of the parallel ridges or the peaks of the diamond knurl. Broadly speaking, if the thickness of an oil film introduced between the two parts under dynamic conditions is less than the peak-to-valley distance, no lubricating effect is accomplished. In some case, the crest of the ridge, or the peak of the diamond knurl, would still have a metalto-metal contact with the mating piece, and therefore wear would occur. In many applications, the mating parts will satisfactorily wear-in: that is, the very high crests or peaks will be knocked off, and the lubricant film will become thicker than the crest-to-valley distance and thereupon function to keep the two members separated under dynamic conditions.

In some applications, however, such methods of finishing flat surfaces, for example on the side plates of a Wankel engine, which are exposed to high speed sliding of a mated part, have proved unsatisfactory. Grinding is accompanied by difficulties in maintaining the desired flatness, surface finish and economy. Machining with a free-flowing abrasive slurry is accompanied by difficulties in maintaining the desired lubrication film, under dynamic conditions. More specifically, by the time the diamond knurl-like surface is worn almost to the grooved shaped valleys to permit lubrication to take effect satisfactorily, the mating part, for example a piston-ring-like seal, is worn out. Moreover, the grooves form continuous passageways which allow the lubricant to escape from the area of the mating surfaces.

Heretofore, special honing techniques have been developed for the satisfactory finishing of the interior or exterior surfaces of cylinders-for example, the cylinder walls of an engine, or the cylindrical surfaces of bearing journals. However, similar methods do not exist for flat surfaces; honing techniques are not adapted to produce the desired overall surface flatness.

SUMMARY OF THE lNVENTlON In the method of the present invention for finishing a flat surface of a work piece, the surface is first machined by urging the work piece under pressure toward a revolving plate which is covered with a free-flowing abrasive slurry. This machining, as mentioned above, produces a diamond knurl-like surface.

Then, in a subsequent step, a stream of beads are directed against the machined surface. In this manner, the peaks of the diamond knurl-like surface are blunted, and the surface is converted to a pocked surface. The surface produced is somewhat like the ball peening done by ancient craftsmen in decorative metal work. When oil is introduced between the finished surface and a sliding mating part, small discrete quantities of oil are entrapped in the pocked surface and thus retained between the two sliding surfaces during the runin period. There are blunted peaks or ridges between one spherical-like indentation and the adjacent indentations; however, the spherical-like indentations are not contiguous and thus do not interfere with the desired entrapment of oil during the run-in period. In addition, the blunted peaks minimize excessive and undersirable wear of the mating part.

The described machining coupled with subsequent bead impingement produces a unique surface finish capable of retaining lubricants under high-speed relative movement between the finished surface and a mating part.

DESCRllPTlON OF THE PREFERRED EMBODIMENT The present invention is concerned with the method of finishing flat surfaces which are exposed to high speed sliding of a mating part. In practicing this method, the flat surface of a work piece is first machined by urging the work piece under pressure toward a revolving plate which is covered with a free-flowing abrasive slurry. The machining operation is carried out in a conventional abrading machine.

Subsequently, a stream of beads are directed against the machined surface. The impingement of beads is carried out, for example, in a conventional sand blast ing machine, wherein beads, such as glass beads, are substituted for sand.

The specific examples below illustrate the method of finishing flat surfaces in accordance with the present invention.

The surfaces of a series of cast iron, steel, and bronze, work pieces were machined in an abrading machine to various preseiected intermediate finishes by urging the work pieces under pressure toward a revolving plate which was covered with a free-flowing abrasive slurry. The root mean square averages (RMA) of the intermediate finishes were measured in millionths of an inch. The machined surfaces of these work pieces were then exposed in a blasting machine to a stream of air-entrained optical crown substantially spherical glass beads directed through a nozzle with an orifice diameter of 0.187 inches for about three seconds per square inch. The weight of the beads passed varied from pounds per hour for the fine beads at low pressure to 325 pounds per hour for the coarse beads at high pressure. With respect to the step of bead impingement, the following variables were altered: the size of the beadsthat is, the diameter measured in inches--; the entrainment air pressure of the beads (nozzle pressure) measured in pounds per square inch; and the angle of impingement of the beads on the surfaces. The root mean square averages (RMS) of the final finishes were measured in millionths of an inch. The intermediate RMS finishes, the bead size, pressure and angle of impingement, and the final RMS finishes, corresponding to the various work piece samples are summarized in the following tables. TABLE 1 (Cast Iron Samples) Cast Iron Comprising:

Carbon 3.24-3.55%

Silicon l.90-2.20%

Manganese 05-07% Phosphorous 0.15% max.

Sulphur 0.l5% max.

Hardness: 170-210 Rockwell B scale lmpinge- Final Intermediate Bead Nozzle ment RMS RMS finish Size Pressure Angle Finish l2 0.023-033 80 90 160 12 023-0033 80 45 120 I2 0023-0033 30 90 7O 12 0.023-0033 30 45 55 15 00017-00035 80 90 100 15 00017-00035 80 45 80 12 00017-00035 90 12 00017-00035 25 45 20 00017-00035 80 90 100 20 0.0017-00035 80 80 20 0.0017-00035 25 90 35 20 00017-00035 25 45 25 20 0023-0033 80 90 I 20 0023-0033 80 45 150 20 0023-0033 30 90 80 20 0023-0033 30 45 65 TABLE ll (Steel Samples) S.A.E. 1020 Cold Rolled Steel lmpinge- Final Intermediate Bead Nozzle ment RMS RMS finish Size Pressure Angle Finish TABLE lll (Bronze Samples) Graphite Bronze With Minimum Hardness of 48 Rockwell B Scale The unique surface finish, which is produced by the above described method of machining with a freeflowing abrasive slurry coupled with subsequent bead impingement, is capable of retaining lubricants under highspeed relative sliding movement between the finished surface and a mating part. Under optimum conditions, the final finished surface is formed with a multiplicity of substantially non-contiguous spherical-like indentations.

The invention claimed is:

l. A method of finishing a flat surface of a work piece comprising the steps of machining the surface by urging the work piece under pressure toward a revolving plate which is covered with a free-flowing abrasive slurry, and subsequently directing a stream of beads against the machined surface.

2. The method of claim 1 wherein the machined surface is machined to an RMS finish in the range of from about 7 to about 45 millionths of an inch.

3. The method of claim 1 wherein the stream of beads is directed against the machined surface until a multiplicity of substantially non-contiguous sphericallike indentations are formed in the surface.

4. The method of claim 1 wherein the size of the beads is in the range of from about 00017 to about 0.033 inches in diameter, the rate of flow of the beads directed against the machined surface is in the range of from about 120 pounds per hour to about 325 pounds per hour, and the beads are directed against the machined surface under a pressure in the range of from about 25 to about psi for about 3 seconds per square inch.

5. The method of claim 4 wherein the machined surface is machined to an RMS finish in the range of from about 7 to about 45 millionths of an inch.

6. A method of finishing a flat surface of a cast iron work piece comprising the steps of machining the surface to an RMS finish in the range of from about 12 to about 20 millionths of an inch by urging the work piece under pressure toward a revolving plate which is covered with a free-flowing abrasive slurry, and subsequently directing a stream of air-entrained glass beads against the machined surface.

7. The method of claim 6 wherein the size of the glass beads is in the range of from about 0.0017 to about 0.033 inches in diameter, the rate of flow of airentrained glass beads directed against the machined surface is in the range of from about pounds per hour to about 325 pounds per hour, and the airentrained glass beads are directed against the machined surface under a pressure in the range of from about 25 to about 80 psi for about 3 seconds per square inch.

8. A method of finishing a flat surface ofa steel work piece comprising the steps of machining the surface to an RMS finish in the range of from about 7 to about 45 millionths of an inch by urging the work piece under pressure toward a revolving plate which is covered with a free-flowing abrasive slurry, and subsequently directing a stream of air-entrained glass beads against the machined surface.

9. The method of claim 8 wherein the size of the glass beads is in the range of from about 0.0017 to about 0.033 inches in diameter, the rate of flow of the airentrained glass beads directed against the machined surface is in the range of from about 120 pounds per hour to about 325 pounds per hour, and the airentrained glass beads are directed against the machined surface under a pressure in the range of from about 25 to about 80 psi for about 3 seconds per square inch.

glass beads is in the range of from about 0.0017 to about 0.033 in diameter, the rate of flow of the airentrained glass beads directed against the machined surface is in the range of from about 120 pounds per hour to about 325 pounds per hour, and the airentrained glass beads are directed against the machined surface under a pressure in the range of from about 25 to about psi for about 3 seconds per square inch. 

2. The method of claim 1 wherein the machined surface is machined to an RMS finish in the range of from about 7 to about 45 millionths of an inch.
 3. The method of claim 1 wherein the stream of beads is directed against the machined surface until a multiplicity of substantially non-contiguous spherical-like indentations are formed in the surface.
 4. The method of claim 1 wherein the size of the beads is in the range of from about 0.0017 to about 0.033 inches in diameter, the rate of flow of the beads directed against the machined surface is in the range of from about 120 pounds per hour to about 325 pounds per hour, and the beads are directed against the machined surface under a pressure in the range of from about 25 to about 80 psi for about 3 seconds per square inch.
 5. The method of claim 4 wherein the machined surface is machined to an RMS finish in the range of from about 7 to about 45 millionths of an inch.
 6. A method of finishing a flat surface of a cast iron work piece comprising the steps of machining the surface to an RMS finish in the range of from about 12 to about 20 millionths of an inch by urging the work piece under pressure toward a revolving plate which is covered with a free-flowing abrasive slurry, and subsequently directing a stream of air-entrained glass beads against the machined surface.
 7. The method of claim 6 wherein the size of the glass beads is in the range of from about 0.0017 to about 0.033 inches in diameter, the rate of flow of air-entrained glass beads directed against the machined surface is in the range of from about 120 pounds per hour to about 325 pounds per hour, and the air-entrained glass beads are directed against the machined surface under a pressure in the range of from about 25 to about 80 psi for about 3 seconds per square inch.
 8. A method of finishing a flat surface of a steel work piece comprising the steps of machining the surface to an RMS finish in the range of from about 7 to about 45 millionths of an inch by urging the work piece under pressure toward a revolving plate which is covered with a free-flowing abrasive slurry, and subsequently directing a stream of air-entrained glass beads against the machined surface.
 9. The method of claim 8 wherein the size of the glass beads is in the range of from about 0.0017 to about 0.033 inches in diameter, the rate of flow of the air-entrained glass beads directed against the machined surface is in the range of from about 120 pounds per hour to about 325 pounds per hour, and the air-entrained glass beads are directed against the machined surface under a pressure in the range of from about 25 to about 80 psi for about 3 seconds per square inch.
 10. A method of finishing a flat surface of a bronze work piece comprising the steps of machining the surface to an RMS finish in the range of from about 10 to 30 millionths of an inch by urging the work piece under pressure toward a revolving plate which is covered with a free-flowing abrasive slurry, and subsequently directing a stream of air-entrained glass beads against the machined Surface.
 11. The method of claim 10 wherein the size of the glass beads is in the range of from about 0.0017 to about 0.033 in diameter, the rate of flow of the air-entrained glass beads directed against the machined surface is in the range of from about 120 pounds per hour to about 325 pounds per hour, and the air-entrained glass beads are directed against the machined surface under a pressure in the range of from about 25 to about 80 psi for about 3 seconds per square inch. 